Electric high frequency signaling system



Dec. 23, 1941. E. c. CORK ETAL' 2,267,445

- ELECTRIC EIGH FREQUENCY SIGNALING SYSTEM Filed A' ri; 21 1938' 9 m 9ngnm INVENTORS EDWARD CFC/L CORK /QSEPH o mwsn' ATTORNEY Patented Dec.23, 1941 ELECTRIC HIGH FREQUENCY SI'GNALING SYSTEM" Edward Cecil Cork,Ealing, Lo'ndtimahdilldsephf Lade Pawsey, Hillingdon, England; assignorsto Electric &, Musical Industries Limited; a

British company Application April 21, 1938, Serial No;- 203,244

In Great Britain May 1, 1937 4 Claims. (01. 250-33) The presentinvention relates to high frequency signaling systems and morespecifically to such systems which? are required to transmit a wide bandof signals.

In the complete specificationof British appli-' cations Nos. 32,339 35and 16,993 36 (Patent No. 469,245), itis explained that in highfrequency signaling systems *such as television systems, it is generallyimpractical to match the transmitter impedance to the input impedance ofthe feeder terminated by the aerial and at the same time obtain thenecessary band width It is pointed outin our prior specification thateven if the reactance of the aerial is small or is neutralized bysuitable circuits the radiation resistance of the aerial will, ingeneral, vary with frequency. The aerialisusl'ially matched to thefeeder at the aerial frequency, but due to the vari-- at ion of theradiation resistance" with frequency;

theaerial will notbe' matched at side band 'fre' quencies. Thismismatching" causes variation in" the 'impedance presentedtothe'transmitter by the aerial system, and hencev'a'riations in theradiated power over the wide band of frequencies constituting the sidebands of a televisiontransmitter. I r

Theph ase angle of current reflected at the aerial end of the feederwhen it reaches the transmitter depends on the length of the feeder Ihaving regard to the width of the band of modu lation frequencies. r

In the aforesaid priorspecification, methods are described ofsubstantially eliminating the variation of the aerial impedance withfrequency so that there is at all relevant frequencies a substantially:constant resistance, by I means of lengths of lineandresonant circuits,sothat it can be connected to the associated transmitter byany length oftransmission line."

Where it is necessary forthe transmitting -sys tem' as awhole to have a,band pass characteriStiCyihB-rCOIlStEtIlt resistance aerial provided bythe methods of -the above applications would be coupledvtothe finaltuned (tank) circuitof the transmitter by a further-tuned circuit. 1Such arfeeder "wto lcompensate for the reactaii'ce' change of -th 'tankcircuit of v the transmitter,- and the 50 "companying drawing in whichothertuned circuit being inserted towards the aerialend of the feeder-tdannul reactance' change due to thefaerial.

Now the relative variations of the impedance of i the aerial and of thetank circuit of the transmitter are of the same order of magnitude andfor this reason in some "cases a simplificaf tion of the band -passarrangement can be, ef-

fected; in that the variation oftheimpedanc'e of r the aerial maybecaused to "appear from "the transmitter as a-reactance variation'ofopposita'f senseto that of the variatio'nin the reactance "of thetransmitter itself, whereby variations I in the impedance of the aerialwith changing fre quency-tend to compensate the variations in thejoutputimpedance of the transmitter} thusfif the length of the feeder iscorrectly choserif the tuned circuits provided inthe feeder 'describedin the prior case may be omitted."

Thus, according to the present invention, 'a radio signaltransmitting'or "receiving arrangement more especially for use on shortwaves is provided comprisinga radio transmitter or receiverso associatedwith an aerial-that for -a' band of frequencies in the pass range ofthe? arrangement, the variationof the-impedance of the aerial as viewedfromthe transmitter or receiver appears as a reactance variationsubstantially opposite in sense'to the variation in the reactance of thetransmitter" or receiver itself, whereby variationsin the impedance ofthe aerial with changing frequency tend to compensate the variationsin'the output impedance" of the transmitter. V

In the preferred arrangement according to the invention the variation ofthe impedance of the aerial as viewed: from transmitterappears as areactance substantially equal in value to the variation of therreactanc'e -of the transmitter over said band of"frequericiesi Incertain circumstances,- in an, arrangement according to-theinvention; atransmitter and its aerial may be, connected without the inter positionof a feeder, or a simple half wave length feeder may be'provided. V i

v The nature'ofthe invention and the method of carrying -the same 'intoeffect; willbe more clearly understoool from the following descrip tion"detaiP reference being made to the Figure 1 shows a schematic diagram ofa bandpass aerial system embodying the invention described in our priorspecification above referred to.

Figure 2 is a similar representation of an arrangment according to thepresent invention.

Figure 3 is an explanatory equivalent circuit of the arrangement ofFigure 2.

Figure 4 is a vector diagram applicable to the arrangement of Figure 2,I

Referring to Figure 1, it will be seen that the output circuit of atransmitter including tube l and tank circuit 2 is shown connected overtransformer 3 to feeder 4 which is directly coupled to dipole aerial 5.In order to compensate for the variation in the reactance of the tankcircuit of the transmitter the secondary winding of the transformer 3 isassociated with condensers 6, which form a resonant combination with thesaid secondary winding, and in order to compensate for the variation inaerial reactance the series tuned combination I of inductive andcapacity elements is provided.

Now the changes in the magnitude of the reactances which are compensatedin the above arr' rangement are of the same order, and in accordancewith the present invention, instead of providing tuned combinations 3,6,6,andl,the length of the feeder is chosen so that the reactive change ofthe aerial may have the effect of a series tuned circuit located at thepoint where the feeder is connected to the transmitter. For example,referring to Figure 2, aerial 5 is'shown connected over feeder 4 totappings on the inductance 3' of the tank circuit 2 of the transmitterthe inductance 3 thus serving as an auto-transformer;

Figure 3 shows the equivalent circuit in'this case. In this figure, thetank circuit 2 of Figure 2 is represented by the tuned circuitcomprising inductance L and capacity C, while L, and C, and R representthe equivalent circuit of the feeder and aerial.

In such, a circuit the constant K band-pass I condition is given by 2= R0 0. and Lcwo2=l where am is the angular velocity, corresponding to themidfrequency f0 of the band.

The circuit comprising elements L1, C1 and R1 The input impedance of thefeeder at any given frequency may be obtained-from the vector diagram ofFigure 4 in which line 00' is a'vectorial representation of thecharacteristic impedance of the feeder which is made equal to RA, andthe vector O'P is the resultant change of impedance due to theincrements of resistance reactance of magnitude OP: the angle 20 isgiven by the equation The aerial impedance viewed from the transmit terwill then be given by the expression Equation 3 can satisfy the bandpasscondition given by Equation 1 if dR 2 dXAF} 2 4712C (4) Now for a halfwave dipole aeria1 with a tuned impedance at mid-band frequency of 72ohms 26 360 dgs. cotis approximately and is approximately Z (GA) whereZOA is the characteristic impedance of the aerial. Hence the value ofZOA for a given value of C can be found and the electrical length offeeder required to produce the apparent rotation of OP is given from theEquation 3 as 2 5 =n 360 cot where n is any integer, including zero. Itis apparent from the form of this equation that the same physical lengthof line approximately satisfies the equation over a band of frequencies.

By way of example, it may be noted that in the case given if C=l09 ,u f.the characteristic impedance of the aerial Z0A=72 ohms, and for 0:440 i,ZoA=400 ohms. These are the approximate limits over which the aerial mayconveniently be varied.

It will be noted that the physical length of line required to satisfyEquation 5 is slightly dependent on frequency, since the electricallength required is constant. In practice, therefore, a single feederwill not be efiective equally at all freplane transmitter. -In manycases, this restric- I tion on the use of theinvention makes itprefer-able to use an arrangement such as that shown in Figure 1 of thedrawing.

Certain special cases of the application of the invention are ofinterest. For example, if the variation of the resistance with frequencynt is zero or negligibly small compared withthe variation an f as forexample in a condensed array, the Equation 4 above becomes and theelectrical length of the feeder required is given by :11. 180 degs.where n is any integer including zero. The simplest case is that inwhich n and consequently 0 is zero, in which case the transmitter wouldbe directly connected to the aerial but if a direct connection isinconvenient,

of conditions'for the application of the inven-v tion may be determinedin the manner described above for any particular case; The inventionwill, of course, be applicable in any case where it is possible totransform the variation of the impedance of an aerial to substantially alinear reactance variation of the correct magnitude.

In the arrangement described above, the feeder is chosen to match theaerial impedance, so that the input impedance to the line is equal tothe tuned aerial resistance. If it is desired to work into a differentimpedance, an unmatched line may be used of suitable impedance andelectrical length, so that it acts as a transformer and produces thedesired bandpass action. The invention is not only applicable at thetransmitting end of signaling system, but is equally applicable to areceiver.

Having now particularly described and ascertained the nature of our saidinvention and in what manner the same is to be performed, we declarethat what we claim is:

1. In combination, an aerial having an impedance comprising a reactancewhich varies with the frequency of a wave applied thereto and asubstantially constant resistance, a feeder con nected to said antennaand a coupling circuit connected to said feeder for coupling a source ofreactance of said output circuit high frequency oscillations to'saidaerial, the reactance of said coupling circuit varying with thefrequency of a wave applied thereto, said feeder having an electricallength equal to a half wave length of the operating frequency so thatthe reactance variation of said aerial viewed from said coupling circuitis substantiallyopposite in sense to the reactance variation. of saidcoupling circuit whereby said variations compensate for each other. I

2. In combination, an aerial having an impedance comprising a reactancewhich varies with the frequency of a wave applied thereto and asubstantially constant resistance, a feeder connected to said antennaand an output circuit of a transmitter connected to said feeder, thereactance of said output circuit varying with the free quency of a waveapplied thereto, said feeder having an electrical length equal to a halfwave length of the operating frequency so that the reactance variationof said aerial viewed from said output circuit is substantially oppositein sense to the reactance variation of said output circuit whereby saidvariations compensate for each other.

3. A broad band transmitting arrangement including an aerial having animpedance comprising a reactance which varies with the frequency of awave applied thereto and a substantially constant resistance, a feederconnected to said antenna and an output circuit of a transmitterconnected to said feeder for coupling said transmitter to said aerial,the reactance of said output circuit varying with the frequency of thewave appearing therein, said feeder having an electrical length equal toa multiple including unity of a half Wave length at the middle frequencyof said band so that the reactance variation of said aerial viewed fromsaid output circuit is substantially opposite in sense to the variationin whereby said variations compensate for each other. i i

4. A broad band antenna arrangement including an aerial having animpedance comprising a reactance which varies with the frequency of a.wave applied thereto and a substantially constant resistance, afeederconnected to said antenna and a coupling circuit connected to saidfeeder for coupling a transducer to said aerial, the reactance of saidcoupling circuit varying with the frequency of the wave applied thereto,said feeder having an electrical length equal to a multiple includingunity of a half wave length at the middle frequency of said band so thatthe reactance variation of said aerial viewed from said coupling circuitis substantially opposite in sense to the variation in reactance of saidcoupling circuit whereby said variations compensate for each other.

EDWARD CECIL CORK.

JOSEPH LADE PAWSEY.

